The present invention relates to silicone compositions for rendering surfaces non-adherent, more particularly to silicone compositions which are curable to form a durable layer on surfaces that renders the surfaces non-adherent, slippery, and easy to release adhesive materials therefrom which are applied thereto.
Conventionally, in order to give release characteristics (the capacity of permitting an adhesive material sticking to a surface to peel off easily) to the surfaces of films and sheets made from cellulose, synthetic resin, and synthetic fiber, the surfaces are coated with a silicone composition which cures to form a film having releaseability. For this purpose, so-called addition reaction-type silicone compositions are commonly used, whose main components are vinyl group-containing organopolysiloxanes and organohydrogenpolysiloxanes having silicon-bonded hydrogens.
The application and curing of such silicone compositions on the surface of a base material in the form of a film or sheet is conducted at temperatures from 100.degree. C. to 200.degree. C. for no longer than 5 to 60 seconds because the base material is usually as thin as 100 .mu.m or even thinner, and is scarcely heat resistive, and also for reasons of economics. However, the known silicone release agents used for rendering surfaces non-adherent are more or less unpredictable in their ability to form a cured film on such surfaces, especially with respect to durability, which is so poor that the releaseability of the film does not persist. In other words, the peel resistance (hereinafter referred to as "peeling strength") tends to become abnormally high, as time goes by. This tendency is the most profound when the adhesive material is acrylic solvent-type.
Another problem arises when the application and curing process is conducted at a relatively low temperature and at a rate that barely allows completion of curing of the silicone composition before the sheet is rolled, i.e., the barely cured film on the sheet tends to stick to itself as well as to the back surface of the sheet as the sheet is rolled into a roll, whereby smooth rolling operation is blocked (hereinafter called "blocking"). Also, when a cured film with a static charge is exposed to the atmosphere for a relatively long period, the charged film adsorbs dust from the air and the resulting dusty film tends to lose much of its ability to release an adhesive material that is applied to it. Conventional silicone release agents are also apt to form a cured film which has poor slip factor and therefore are not suitable for labels for music tape cassettes, which require good slip factor.
When the manufacturing line for applying silicone composition to the base sheet or film is arranged such that the cured film of the silicone composition comes in contact with receiving spools or guide rollers, a cured film with poor slip factor tends to cause the flow of the sheet or film to stagnate at the spools and rollers, which also results in contamination of the spools and rollers.
In order to solve these problems, a method was proposed, according to which, the amount of the platinum-containing catalyst contained in the addition reaction-type silicone composition is increased thereby rendering it possible to shorten the heating time required for curing the coating film. However, the silicone compositions formulated in this manner have reduced pot life, the cured film obtained from this composition is chemically so unstable that it readily undergoes degradation when exposed to air, and its slip factor is yet far from desirable. Moreover, the increased amount of expensive platinum-containing catalyst leads to poor economy.
It has also been examined whether admixing the compositions with a dimethylsiloxane oil or gum, a silicone oil modified by polyether, a surface active agent, wax, etc. would bring about positive results. Unfortunately, the results were more or less negative, i.e., the resulting silicone compositions proved to be less curable and the part of the composition which fails to cure sufficiently on the base sheets blooms (emerges from the cured surface) so that when a pressure sensitive adhesive material is pressed on the cured film surface, the insufficiently cured composition adversely affects the residual adhesive strength of the adhesive material, i.e., the strength of the adhesion of an adhesive material measured after it is peeled off a surface to which it has been pasted.